1. Field of the Invention
The present invention relates to an oscillation device for detecting a temperature at which a quartz-crystal oscillator is placed, and for conducting temperature compensation of an output frequency based on a result of the temperature detection.
2. Description of the Related Art
Normally, when a quartz-crystal oscillator is incorporated in an application to which quite high frequency stability is required, an oven controlled Crystal Oscillator (OCXO) is generally used, but the OCXO is formed of a large-scale device, and thus has a large power consumption. For this reason, it is considered to use a Temperature compensated crystal oscillators (TCXO) with a simple structure and small power consumption, but the TCXO has a disadvantage that a frequency stability with respect to a temperature is inferior to that of the OCXO.
FIG. 19 shows a general structure of the TCXO. Reference numeral 90 denotes a quartz-crystal oscillator and reference numeral 91 denotes an oscillation circuit, and by changing a control voltage supplied from a control voltage generating unit 93 to a voltage variable capacitance element 92, a capacitance of the voltage variable capacitance element 92 is controlled to adjust an oscillation frequency (output frequency).
A frequency of the quartz-crystal oscillator 90 changes in accordance with a temperature, so that the control voltage generating unit 93 compensates the control voltage in accordance with a temperature detected by a temperature detector 94. Concretely, a cubic function, for example, being a function in which a frequency-temperature characteristic of the quartz-crystal oscillator 90 is normalized by a reference temperature is stored in a memory 95, and a frequency corresponding to a temperature detection value is read based on this function (frequency-temperature characteristic). Specifically, how much a frequency at a temperature at that time is deviated from a frequency at the reference temperature is read, and a control voltage corresponding to a deviation amount of the frequency is set as an amount of temperature compensation, and is subtracted from a control voltage corresponding to the frequency at the reference temperature.
However, when the temperature compensation control is tried to be finely conducted, an amount of data for specifying the function of the frequency-temperature characteristic becomes large, and a large-capacity memory is required as the memory 95, resulting in that the memory 95 becomes expensive. Further, a thermistor is normally used as the temperature detector, so that even when the amount of data is enlarged, an improvement in frequency accuracy cannot be expected due to a limit in detection accuracy of the temperature detector.
Further, since the temperature detector 94 and the quartz-crystal oscillator 90 are disposed at different positions, it is not possible to correctly obtain actual temperature information of the quartz-crystal oscillator 90, and from this point as well, the improvement in frequency accuracy cannot be expected.
In FIG. 2 and FIG. 3 of Patent Document 1, it is described to structure two quartz-crystal oscillators (quartz-crystal resonators) by providing two pairs of electrodes on a common quartz-crystal piece. Further, paragraph 0018 describes that a frequency difference appears between the two quartz-crystal oscillators in accordance with a temperature change, and measuring of this frequency difference is equivalent to measuring of temperature. Further, a relation between this frequency difference Δf and an amount of frequency to be compensated is stored in a ROM, and an amount of frequency compensation is read based on Δf.
However, as described in paragraph 0019, this method is required to adjust the quartz-crystal oscillators so that, regarding a desired output frequency f0 and respective frequencies f1, f2 of the two quartz-crystal oscillators, a relation of f0≈f1≈f2 is satisfied, and thus there are problems that a manufacturing process of the quartz-crystal oscillators becomes complicated, and besides, it is not possible to achieve high yield. Furthermore, as shown in FIG. 4 of Patent Document 1, clocks being frequency signals from the respective quartz-crystal oscillators are counted for a given period of time, and a difference between the clocks (f1−f2) is determined, so that a detection accuracy directly influences on a detection time, which makes it difficult to realize high-accuracy temperature compensation.
[Patent Document 1] Japanese Patent Application Laid-open No. 2001-292030